There are a number of commercially available products which provide sensing, control, and communications in a network environment. These products range from elaborate systems having a large amount of intelligence to simple systems having little intelligence. By way of example, such a system may provide control between a light switch and a light. When the light switch is operated, a digital code pattern is transmitted by one cell and is received by another cell located near the light. When the code is received, it is interpreted and subsequently used to control the light. Such a system, comprising a network of intelligent cells in which the cells communicate, control and sense information, is described in U.S. patent entitled, "Network and Intelligent Cell for Providing Sensing, Bidirectional Communications and Control", U.S. Pat. No. 4,969,147, issued Nov. 6, 1990, which is assigned to the assignee of the present invention.
The transmitting and receiving of digital data can be performed by a series of transceivers, each of which is connected to an individual cell of a network. These transceivers may communicate with one another in numerous different ways over various media and at many different baud rates. For example, the transceivers could be connected to standard communications lines, such as twisted pair lines, fiber optic cables, and coaxial cables. Indeed, even power lines have been employed as a transmission medium by implementing spread spectrum techniques.
In order to minimize costs, the same transmission lines coupling the various cells can be used to provide a medium for the transmission of data between the cells as well as for supplying power to the cells. In this scheme, duplicate communications lines and power lines are not required. Instead, a single set of transmission lines perform the dual functions of conveying power and data amongst the cells. This optimization can be realized by interposing a source power isolator between the power source and the cells. The source power isolator provides isolation for the relatively high data frequencies while also passing direct current (DC) on transmission lines for powering the cells. In the prior art, source power isolators were typically comprised of a pair of inductors.
FIG. 1 shows a typical prior art source power isolator 101. The source power isolator 101 is comprised of two inductors 102 and 103, which provide the necessary impedance for isolation at the high data frequencies. This impedance must be made relatively high. Otherwise, the data signals generated by the cells 106-108 will fail to create an adequate voltage across the transmission lines 104 and 105. Furthermore, high impedance is needed for loading the transmission lines 104 and 105 with its characteristic impedance (e.g., 50 to 300 ohms).
There are numerous problems associated with these prior art source power isolators. Several of the problems are attributable to the flaws inherent to inductors. First, inductors alone do not provide a controllable DC impedance. For certain power supplies (e.g., switching-power supplies), it is desirable to have a controllable DC impedance during power-up of the communications network. This is because the negative resistance resulting from constant cell power and/or the total effective capacitance that must be charged at system power-up might result in the activation of the source power supply's internal protection circuitry. In turn, this might result in a continuous retry to start, a hang-up on the "foldback" curve, or a failure of the source power supply.
Second, inductors alone cannot provide a voltage drop, as might be required, to drop from a common source voltage (e.g., +48 volts) to below a safety limit (e.g., +42.4 volts). Third, dangerous and damaging overvoltage transients are sometimes created during accidental or momentary disruptions in the transmission line circuit. Consequently, additional safeguarding measures are often required. And fourth, short circuits across the transmission lines and from a transmission line to ground, necessitates other safeguarding measures. Simple inductors cannot limit these overcurrent conditions
Another problem with typical prior art source power isolators is that safety agencies, such as Underwriters Laboratories (UL), may require "single fault tolerance" for protection against overvoltage output situations. Furthermore, as the data rates drop and as DC line currents increase, larger and more expensive source power isolators are required.
Therefore, there is a need in the prior art for a protected source power isolator which provides a constant start-up current at the rated capacity of the system or power source and whose output voltage is limited to fall within safety guidelines. It would be preferable if such an isolator had a delay to allow time for the source power supply to reach full output voltage during system power-up. It would also be preferable if the isolator could preclude power source retry, hang-up, or other failures.